The built-in PDF viewers in the Chrome web browser and Mozilla Firefox are convenient because you don't need to install another program to view PDFs. However, if you prefer using an external PDF reader, you can disable the built-in browser viewers.

In Firefox, go to Options > Applications, then find "Portable Document Format (PDF)" and click on the drop-down menu to choose an option other than Firefox.

In Chrome, enter, "chrome:plugins" in the address bar, then click the Disable link under the "Chrome PDF Viewer" plugin entry.